1. Field of the Invention
This invention relates to an infrared-to-visible up-conversion material capable of emitting a reradiated light (fluorescence) containing spectral components in the visible range upon irradiation by infrared light. In particular, it is concerned with an infrared-to-visible up-conversion material which can be applied to an infrared light identification element having a useful conversion efficiency and sensitivity to infrared light in the wavelengths of 1.5 .mu.m band, 0.98 .mu.m band and 0.8 .mu.m band without a necessity of pre-excitation of the material.
2. Description of the Related Art
Recently demands for elements for detecting infrared light are increasing. Applications include detection of a beam position of emitting elements such as laser, LD, LED, etc., pattern identification of mode shapes, etc., searching for a broken point in an optical fiber cable, etc. Systems available for these applications can roughly be classified into a pure electric system, e.g. optical power meter using a semiconductor photodiode, and a visually detectable system using vidicon, image intensifier, IR excitable phospor, etc.
The pure electric system has high sensitivity, but has a disadvantage that visual detection is impossible. On the other hand, the vidicon or image intensifier has a disadvantage that the sensitivity is insufficient and the production cost is higher. Accordingly, a system has been considered promising using an infrared-to-visible up-conversion material such as infrared-excitable phosphors, having visual detectability, relatively high conversion efficiency and sensitivity and lower expense.
A phosphor is generally excited by a suitable excitation source. In the emitted light from the excited phosphor, various spectroscopic distributions can be provided depending upon the variety of the phosphor. Therefore, the phosphor can be applied to various application fields by combination with a suitable excitation source. Above all, a material called "infrared-excitable phosphor" is known as a wavelength up-conversion phosphor of from infrared to visible light. For the anti-Stokes-wise wavelength conversion of from such infrared light to visible light having a markedly large photon energy, it is required to well choose the relationship between the property and excitation wavelength of a material.
As the infrared-to-visible wavelength up-conversion material of the prior art, an IR Sensor Card (commercial name, made by QUANTEX Co.) has well been known. This IR Sensor Card emits red light or blue-green light under infrared irradiation depending on the material of the phosphor. For this sensor, a previous excitation is required before irradiation of infrared light (although possible by room light) and infrared light-excitation is first possible through this previous excitation process. When infrared light is continuously irradiated on this infrared light detecting element, however, there arises a problem that the conversion efficiency is varied with passage of time, although reversible, and the emission intensity of visible light is gradually lowered.
On the other hand, infrared-to-visible wavelength up-conversion materials not needing previous excitation have been reported, typical of which are YF.sub.3 ; Er, Yb; Y.sub.3 OCl.sub.7 ; Er, Tb (H. Kuroda et al. J. Phys. Soc. Jpn.; Vol. 33, No. 1, 1972, pp 125-141), NaLnF.sub.4 ; Er, Yb (Ln: Y; Gd, La) (T. Kano et al. J. Electrochem. Soc., Vol. 119, No. 11, 1972, pp 1561-1564); Ba Y.sub.2 F.sub.8 : Er, Yb (Y. Mita et al. Appl. Phys. Lett., Vol. 23, No. 4, 1973, pp. 173-175), (PbF.sub.2 --GeO.sub.2): Er, Yb, (PbF.sub.2 --GeO.sub.2): Tm, Yb (F. Auzel et al., J. Electrochem. Soc., Vol. 122, No. 1, 1975, pp. 101-107). These materials utilize the multiphoton excitation of rare earth ions (mainly Er.sup.3+ ion).
However, the above described materials utilizing the multiphoton excitation of rare earth ions each have a low conversion efficiency of infrared light and low sensitivity. As described above, the prior art has the disadvantages that a previous excitation is required before irradiation of infrared light and when infrared light is continuously irradiated, the emitting intensity of visible light is gradually lowered. Even if a previous excitation is not required, the conversion efficiency and sensitivity are low.